Mutualism Involving Three-Toed Sloths
WHAT ARE SLOTHS?
Sloths are the slowest mammals on earth. In fact, the well know adjective, slothful which means lazy or idle, is named for these creatures, that do pretty much everything at an almost comically slow pace.
One of the reasons for the sloth’s slow paced life is its diet. The three-toed sloth lives mainly on leaves from the rain forest canopy. These leaves are toxic and provide very little energy as a food source. To aid the sloth in digesting these leaves, it is equipped with a multi-chambered stomach much like a cow’s. The process of digestion for the sloth can take up to a month. If the digestion was any faster, the sloths might be poisoned by the leaves. Sloths spend up to 70% of their time resting. Not because they are lazy – their method of digestion just requires it.
Although the sloth’s appearance and mannerisms may seem odd, much of that is because it has evolved to expend the minimum amount of energy possible.
Three-toed sloths are designed for life hanging in the trees. They do almost everything hanging upside. This includes eating, sleeping, mating and even giving birth. This body structure allows the sloth to function with only a quarter of the muscle mass of other mammals its size. Hence, the metabolism of the three-toed sloth is much lower than for other mammals.
But the sloth’s evolutionary deviation comes at a price. First is the slow speed at which it carries out its entire existence. In a world of predators and prey, slowness is rarely a positive attribute.
Another issue for the sloth is that, with its lack of muscle, it is ill-equipped to walk on all fours. For those rare times that a sloth descends the safety of the trees, it must awkwardly drag itself along the ground, laying on its belly.
When the three-toed sloths come down from the trees, they are susceptible to predators such as snakes, speckled owls and harpy eagles. So what would cause the sloth to risk peril when it is fully capable to do everything it needs from the safety of the canopy?
Once a week, the three-toed sloth leaves the tree cover in order to urinate and defecate (digging a hole first, and covering it afterwards). Its cousin, the two-toed sloth is able to perform these functions from the safety of the trees. And it is said that, during storms, three-toed sloths have been witnessed defecating from the trees. So, if the sloth is fully capable of remaining off the ground, why would it expend so much energy to climb down from the trees? This seems to go against everything evolution has provided for it as far as conserving energy as well as self preservation.
One theory is that this ritual is necessary in order to attract a mate. The sloth’s sense of smell is more acute that its hearing or eyesight. There is no doubt that many species are willing to risk safety for the purpose of breeding.
There is another theory that is equally legitimate, and this one finally brings us to the subject of mutualism.
There are some moth species which exclusively colonize the fur of sloths. When the sloths descend the trees to defecate, the female moths lay their eggs in the sloth’s dung. When the new adult moths emerge from the dung, they fly up into the canopy to mate within the sloth’s fur.
The presence of moths provides a nitrogen-rich environment in the sloths fur. These added nutrients in the fur promote the growth of certain algae.
The sloths benefit from the presence of algae in two ways.
First, the algae becomes a fat-rich supplement in the sloth’s otherwise poor diet.
A second advantage of the algae is that it tints the fur of the sloth, providing a degree of green camouflage which can be very effective in the forest canopy.
Two-toed sloths can also harbor moth colonies without taking the risk of descending the trees. But the amount of moths, nitrogen and algae has been found to be significantly lower than for the three-toed sloths.
Whether the three-toed sloths descend the trees as part of their mutualistic obligation or not is unclear. But what is clear is that the lives of the sloth, the moth and the algae are woven into an intricate web of interdependence.
Banner photo: Christian Mehlführer, via Wikimedia Commons. Distributed under a CC BY 2.5 license.